Sol-gel synthesis of thin solid Li7La3Zr2O12 electrolyte films for Li-ion batteries

• We report on a sol-gel processing method for the synthesis of garnet-type Yttrium-doped cubic Li7La3Zr2O12 (LLZ) samples
• Powder, pellets and thin films were successfully obtained
• The highly conductive cubic LLZ phase was achieved after the introduction of suitable RTP heating programs in argon
• Prototypes of thin film half-cells with LiCoO2 and LLZ single phase layers were developed

The application of a solid state electrolyte layer could greatly improve current Li-ion batteries in terms of safety and reliability. Garnet-type Li7La3Zr2O12 (LLZ) appears as a candidate material, since it shows the highest reported Li-ion conductivity of all oxide ceramics at room temperature (σ > 10− 4 S cm− 1) and at the same time chemical stability against lithium. In this paper, a sol-gel process is presented for fabricating homogeneous thin film LLZ layers. These layers were deposited using dip-coating and spin-coating methods. A stable Yttrium-doped Li-La-Zr-based sol with a particle size of d50 = 10 nm was used as coating liquid. Successful deposition of such layers was accomplished using a sol concentration of 0.04 mol/l, which yielded for each coating step a layer thickness of ~ 50 nm. The desired single phase LLZ material could be obtained after thermal treatment at 800 °C for 10 min in Argon. Ionic conductivity of the layers was demonstrated with impedance spectroscopy. Continuing work on the development of half-cells is also presented. Half-cells which contain the novel LLZ electrolyte layer, a LiCoO2 cathode and a steel support were synthesized and investigated. Of considerable importance was the prevention of Lanthanum diffusion and the formation of non-conductive phases (e.g. La2Li0.5Co0.5O4) at the required heating temperature of 800 °C. It is shown that these unwanted processes can be prevented and that a structure with a single phase LLZ and LiCoO2 layer can be obtained by modifying the heating program to a rapid thermal treatment (10 K/s, 800 °C, no holding time).